Planetary researchers on NASA’s Cassini mission have published a new study describing the process that drives and sustains long-lived geysers on Enceladus, Saturn’s sixth largest, icy moon.
As it swooped past the south pole of Enceladus in July 2005, Cassini acquired high resolution views of this puzzling ice world. From afar, the moon exhibits a bizarre mixture of softened craters and complex, fractured terrains. This large mosaic of 21 narrow-angle camera images have been arranged to provide a full-disk view of the anti-Saturn hemisphere on Enceladus. This mosaic is a false-color view that includes images taken at wavelengths from UV to IR portion of the spectrum. In false-color, many long fractures on Enceladus exhibit a pronounced difference in color (represented here in blue) from the surrounding terrain. Image credit: NASA / JPL / Space Science Institute.
Cassini has observed geysers erupting on Enceladus since 2005, but the process that drives these long-lived eruptions has remained elusive.
Dr. Allan Rubin from Princeton University and Dr. Edwin Kite from the University of Chicago have now pinpointed a mechanism by which cyclical tidal stresses exerted by Saturn can drive the eruptions.
“Cassini data have strongly indicated that the cryovolcanic plumes of Enceladus probably originate in a biomolecule-friendly oceanic environment,” the researchers said.
“One of the problems that attracted our attention was the anomalous tidal response of the Enceladus eruptions.”
The eruptions reach their peak about 5 hours later than expected, even when taking into account the 40 minutes needed for the erupted particles to reach the altitude at which Cassini can detect them.
Other planetary researchers had previously suggested reasons for the lag, which included a delay in the eruptions as well as a squishy, slowly responding ice shell.
“The new proposal is really a way to get a delay in the eruptions. You really don’t need to propose any terribly squishy ice shell to do it,” said Dr. Carolyn Porco, head of Cassini’s imaging science team and a leading scientist in the study of Enceladus.
Dr. Kite and Dr. Rubin also wanted to know why the icy moon maintains a base level of cryovolcanic activity, even when at that point in its orbit where Enceladus’ fissures should clamp shut and curtail the eruptions.
Other key questions raised by the scientists: “why does the volcanic system generate five gigawatts of power instead of a lot more or a lot less? And why don’t the eruptions frost over or freeze over?”
Their model of the Enceladus plumbing system seems to answer them all.
The model consists of a series of nearly parallel, vertical slots that reach from the surface down to the water below.
“We found that a simple model in which the erupting fissures are underlain by slots that connect the surface to the ocean can explain the observations,” Dr. Kite and Dr. Rubin said.
“In the model, the slots are mostly filled with water, and Saturn tides drive turbulent water flow in the slots whose dissipation produces enough heat to keep slots open.”
“In turn, long-lived water-filled slots drive a volcano-tectonic feedback that buffers the rate of volcanism to approximately the observed value.”
The results, published online this week in the Proceedings of the National Academy of Sciences, also suggest that the ocean-surface connection on Enceladus may be sustained on million-year timescales.
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Edwin S. Kite & Allan M. Rubin. Sustained eruptions on Enceladus explained by turbulent dissipation in tiger stripes. PNAS, published online March 28, 2016; doi: 10.1073/pnas.1520507113
